Digital watermarking is a process for modifying physical or electronic media to embed a machine-readable code into the media. The media may be modified such that the embedded code is imperceptible or nearly imperceptible to the user, yet may be detected through an automated detection process. Most commonly, digital watermarking is applied to media signals such as images, audio signals, and video signals. However, it may also be applied to other types of media objects, including documents (e.g., through line, word or character shifting), software, multi-dimensional graphics models, and surface textures of objects.
Digital watermarking systems typically have two primary components: an encoder that embeds the watermark in a host media signal, and a decoder that detects and reads the embedded watermark from a signal suspected of containing a watermark (a suspect signal). The encoder embeds a watermark by altering the host media signal. The reading component analyzes a suspect signal to detect whether a watermark is present. In applications where the watermark encodes information, the reader extracts this information from the detected watermark.
Several particular watermarking techniques have been developed. The reader is presumed to be familiar with the literature in this field. Particular techniques for embedding and detecting imperceptible watermarks in media signals are detailed in the present assignee's co-pending application Ser. No. 09/503,881 (Now U.S. Pat. No. 6,614,914).
The invention relates to encoding and decoding auxiliary signals in a media signal, such as a still image, video or audio signal, using a wavelet or subband decomposition of the signal. One aspect of the invention is a method of embedding an auxiliary signal into a media signal so that the auxiliary signal is substantially imperceptible. This method performs a wavelet decomposition of the media signal, and embeds a watermark orientation signal into the wavelet decomposition such that the watermark signal forms a pattern in a transform domain used to perform synchronization of the watermark. The pattern has attributes used to determine orientation of the watermark in a geometrically distorted version of the media signal. The attributes carry two or more auxiliary message symbols in the watermark, and the attributes both carry the two or more auxiliary message symbols and form the pattern used to perform synchronization of the watermark.
Another aspect of the invention is a method of detecting an auxiliary signal embedded in a media signal, where the auxiliary information is substantially imperceptible in an output form of the media signal. This method performs a wavelet decomposition of the media signal into two or more levels of resolution. The method synchronizes a reference signal with the wavelet decomposition of the media signal to determine orientation of the auxiliary signal in the media signal. The auxiliary signal forms a pattern in a transform domain used to perform synchronization, and the pattern has attributes used to determine orientation of the watermark in a geometrically distorted version of the media signal. The attributes carry two or more auxiliary message symbols in the watermark, and the attributes both carry the two or more auxiliary message symbols and form the pattern used to perform synchronization of the watermark.
Another aspect of the invention is an alternative method of encoding an auxiliary signal in a media signal. This method performs a wavelet decomposition of the media signal into two or more levels of resolution, including an approximate level and one or more higher resolution levels. It then modifies the approximate level to encode an auxiliary signal such that the modification is substantially imperceptible in an output form of the media signal. The auxiliary signal serves to carry two or more message symbols and forms peaks in a transform domain used to perform synchronization of the auxiliary signal.
Another aspect of the invention is a method of detecting an auxiliary signal embedded in a media signal, where the auxiliary information is substantially imperceptible in an output form of the media signal. This method performs a wavelet decomposition of the media signal into two or more levels of resolution, including an approximate level and one or more higher resolution levels, and detects the auxiliary information from the approximate level. This method detects a pattern of peaks in a transform domain to perform synchronization of the auxiliary signal.
The disclosure further describes an auxiliary signal that includes a signal used to determine orientation of the auxiliary signal called the watermark orientation signal. This orientation signal has attributes used to determine orientation of the auxiliary signal in a geometrically distorted version of the media signal. The orientation signal may carry a message comprising one or more symbols (e.g., binary or M-ary symbols) of information. Alternatively, a separate watermark message signal may carry the message. When embedded as separate signals, the orientation and message signal components of the watermark may be orthogonal to one another. In a wavelet decomposition of the host signal, the watermark embedder may insert the message and orientation signal components in separate subbands. A compatible detector conelates a reference watermark orientation signal with the wavelet decomposition of the media signal to determine orientation of the auxiliary signal in the media signal.
Additional features of the invention will become apparent with reference to the following detailed description and accompanying drawings.